Solution_2018_01.swift

import Foundation

struct Solution_2018_01: Solution {
    var input: Input
    func run() throws {
        let frequencies = try input.get()
            .components(separatedBy: .whitespacesAndNewlines)
            .compactMap({ Int($0) })

        // ------- Part 1 -------

        let part1 = frequencies.reduce(0, +)
        print(part1)

        // ------- Part 2 -------

        var current = 0
        var index = 0
        var visited = Set<Int>()

        while visited.insert(current).inserted {
            current += frequencies[index]
            index = (index + 1) % frequencies.count
        }

        let part2 = current
        print(part2)

        /*
         Originally my solution was to construct an infinite lazy sequence : D

         let repeatedFrequencies = sequence(first: (0, frequencies[0]), next: { (idx, _) in
             let next = (idx + 1) % frequencies.count
             return (next, frequencies[next])
         }).lazy.map({$0.1})

         var current = 0
         var visited = Set<Int>([current])

         _ = repeatedFrequencies.first(where: { change in
             current += change
             return !visited.insert(current).inserted
         })

         let part2 = current
        */

        // ------- Test -------

        assert(part1 == 425, "WA")
        assert(part2 == 57538, "WA")
    }
}